The present invention relates to a hydraulic drive system for a crawler-mounted construction vehicle such as a hydraulic excavator. More particularly, the present invention concerns a hydraulic drive system having pressure balance valves for effecting load-compensating control of the flow rates of hydraulic fluid supplied to a pair of traveling motors that are respectively adapted to drive the left and right crawler belts, i.e., traveling means for the crawler-mounted construction vehicle.
As disclosed in U.S. Pat. No. 4,425,759, a known hydraulic drive system for a crawler-mounted construction vehicle comprises a hydraulic pump, left and right traveling hydraulic motors driven by a hydraulic fluid discharged by the hydraulic pump, flow control valves for respectively controlling the flow rates of the hydraulic fluid from the hydraulic pump to the traveling motors, a pump regulator for effecting control during the driving of the left and right traveling motors in such a manner that the discharge pressure of the hydraulic pump becomes higher by a fixed value than the higher one of load pressures of the left and right traveling motors, and pressure balance valves for respectively controlling the rates of flow through the flow control valves during the driving of the left and right traveling motors in such a manner that the load pressure of the traveling motors does not affect the rates of flow through the flow control valves. Control of the rates of flow through the flow control valves effected by these pressure balance valves is referred to herein as load-compensating control.
In the hydraulic drive system disclosed U.S. Pat. No., 4,425,759, the arrangement is such that the pressure balance valves are disposed downstream of the associated flow control valves, and the pressure downstream of the flow control valves acts in the valve opening direction, while the higher one of load pressures of the left and right traveling motors acts in the valve closing direction.
Thus, by providing the pressure balance valves for effecting the load-compensating control of the rates of flow through the flow control valves for the left and right traveling motors, even if there is any differential between the load pressures of the left and right traveling motors during traveling, it is possible to effect distribution of fluid flow corresponding to the ratio of the valve openings (demand flow rates) of the flow control valves for left and right traveling motors. In case of steering the crawler belts to change the advancing direction of the vehicle, the hydraulic fluid is supplied positively to the higher load side traveling motor associated with the outwardly turning crawler belt, making it possible to effect an intended change in the course. In addition, when the vehicle is made to travel straight, even if the resistance to which the left and right crawler belts are subjected differs, control is effected in such a manner that the rates flow through the flow control valves become equivalent, thereby effecting straight traveling.
With this conventional system, however, since the straight traveling is effected by conducting the load-compensating control of the rates of flow through the flow control valves by means of the pressure balance valves, variations in the performance of hydraulic devices such as the flow control valves, the pressure balance valves, etc., that are ascribable to fabrication errors affect the straight traveling characteristics. For this reason, it has been necessary for the operator to adjust the advancing direction while viewing the actual traveling direction. In addition, in cases where the strokes of control levers change slightly and the openings of the flow control valves is thereby changed, the rates of flow through the flow control valves also change, thereby hampering the straight traveling characteristics. Consequently, it has been necessary for the operator to pay utmost attention to ensure that the strokes of the control levers do no change by the slightest degree. Thus, with the conventional system, there has been the problem that substantial labor is required of the operator in effecting the straight traveling of the vehicle, increasing the operator's fatigue.
In addition, U.S. Pat. No. 4,535,809 discloses a system which is not directly related to a hydraulic drive system for a crawler-mounted construction vehicle, but in which a flow control valve is arranged by a seat valve assembly which comprises a seat-type main valve, a pilot circuit associated with the main valve, a pilot valve disposed in this pilot circuit and adapted to control the operation of the main valve, and a pressure balance valve disposed in the pilot circuit and adapted to effect the load-compensating control of the rate of flow through the pilot valve.
In addition, DE-A3422165 discloses a pressure balance valve which is disposed upstream of a flow control valve and which causes the discharge pressure of a hydraulic pump and the outlet pressure of the flow control valve to act in the valve opening direction and causes the maximum one of load pressures of a plurality of actuators and the inlet pressure of the flow control valve to act in the valve closing direction.